This application claims priority to EP 00311670.4 filed Dec. 22, 2000, herein incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates generally to a lithographic projection apparatus and more particularly to lithographic projection apparatus including a contaminant removal system.
2. Description of the Related Art
Lithographic projection apparatus in accordance with the present invention generally include a radiation system for supplying a projection beam of radiation, a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate, and a projection system for projecting the patterned beam onto a target portion of the substrate.
The term “patterning structure” as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.        A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning structure can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.        
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
To reduce the size of features that can be imaged using a lithographic projection apparatus, it is desirable to reduce the wavelength of the illumination radiation. Ultraviolet wavelengths of less than 180 nm are therefore currently contemplated, for example 157 nm or 126 nm. Also contemplated are extreme ultraviolet (EUV) wavelengths of less than 50 nm, for example 13.5 nm. Suitable sources of UV radiation include Hg lamps and excimer lasers. EUV sources contemplated include laser-produced plasma sources, discharge sources and undulators or wigglers provided around the path of an electron beam in a storage ring or synchrotron.
In the case of EUV radiation, the projection system will generally consist of an array of mirrors, and the mask will be reflective; see, for example, the apparatus discussed in WO 99/57596.
However, apparatus which operate at such wavelengths are significantly more sensitive to the presence of contaminant particles than those operating at higher wavelengths. Contaminant particles such as hydrocarbon molecules and water vapor may be introduced into the system from external sources, or they may be generated within the lithographic apparatus itself. For example the contaminant particles may include debris and by-products that are sputtered loose from the substrate, for example by an EUV radiation beam, or molecules produced through evaporation of plastics, adhesives and lubricants used in the apparatus.
These contaminants tend to adsorb to the optical components in the system and cause a loss in transmission of the radiation beam. When using 157 nm radiation, a loss in transmission of about 1% is observed when only one or a few monolayers of contaminant particles form on each optical surface. Such a loss in transmission is unacceptably high. Further, the uniformity requirement on the projection beam intensity for such systems is less than 0.2%. Localized contamination can cause this requirement not to be met.
There is also a risk that the adsorption of contaminant particles on the surface of the optical components, or within the optical surface in the case of a porous surface, e.g. an anti-reflection coating, may cause damage, for example cracking, to the optical components themselves. Such damage can occur if the optical components are suddenly irradiated with UV radiation, for example 157 nm radiation, at full power. The irradiation will cause rapid evaporation of the smaller contaminant particles, such as water molecules, which are trapped within said porous surface of the optical component, hence damaging the optical surface itself. Such damage is extremely costly and even a minimal risk of this occurring should be avoided. It is therefore desired that the optical components in a lithographic apparatus are kept as free of contaminants as possible.
Previous methods for cleaning optical components include, for example, the use of ozone as a cleaning material, its activity being increased by the presence of UV radiation. However, such cleaning methods are very harsh and may damage the optical component to be cleaned, particularly at its surface. In particular, the mask, which generally comprises a Teflon® (PTFE)-based or other organic material, can be damaged by the use of such methods.
A further problem associated with apparatus for use with EUV radiation relates specifically to the presence of water molecules in the apparatus. Typically, the high vacuum systems required for operation in EUV lithography tools contain a high partial pressure of water. However, the presence of water in combination with EUV radiation has a tendency to cause oxidation of mirrors. This is an irreversible and highly damaging process and leads to a very significant loss in mirror reflection. Due to the restrictively high cost of replacing the mirrors, this ultimately leads to the operation of the lithographic tool with poor reflection levels and therefore a reduction in productivity. The reduction in lifetime of the mirrors which also results is a further distinct economic disadvantage.
Oxidation protection cap layers have been suggested as a potential solution to this problem. However, very few successful results in this area have so far been demonstrated. No protective method has yet been found which can increase the lifetime of mirrors used in EUV systems to greater than 15 hours. This is considerably lower than the 10,000 hours that are desired.